Jurgen;
To make the fronds or "leaves" of the palm tree you can use a few methods.
1. Buy a photo-etched set - (Expensive if you're making a number of trees)
2. Use lead foil or foil from wine and liquor bottles trace the shape of the fronds and cut with scissors and/or a hobby knife
3. Use card stock or a heavy guage paper and cut the shape with scissors and/or a hobby knife
4. Use feathers and trim the edges to simulate the shape of the fronds paint as required
5. Visit a pet store that sells aquarium supplies and purchase various "palm" looking plastic plants. Paint and shade as required
6. Visit a florist or shop that sells dried or plastic flower arrangement supplies. Purchase palm looking plastic plants. Paint and shade as required.
7. Check on-line hobby stores for Preiser or Verlinden kits in these scales these kits may still be available.
While #2,#3 and #4 are tedious they will produce good results for minimal cost. #1, #5 and #6 will also give you good results however these methods can add up if you're making a number of trees. #7 is the most expensive but least time consuming in building.
For sandbags in either 1:72 or 1:48 the first principle is size. Use a standing figure in the scale you intend to build in. A full scale sandbag typically reaches the mid-calf of a 6 foot man (I know first hand I have filled a lot of them) The width across the surface of the sandbag when on a flat surface is equal to the width of the figures legs tightly together when standing. The thickness of the bag in height when on a flat surface would be equal to the height of the top of the figures boots. I hope these references make sense. With these in mind you should be able to guage a scale sandbag for either 1/72 or 1/48 scale.
Material for making scale sandbags. In 1/72 or 1/48 scale it would probably be best to use epoxy putty (Milliput or Duro or similar) Once you have the putty simply roll it to the first reference (mid-calf of your figure) once you have the roll of putty at this uniform thickness, take a hobby knife and slice "bags" off the roll at the third reference the height of your figures boot top. Now you can begin to build your retaining wall etc. forming each bag into the next as you go. Include some ammunition crates or similar items at corners or at the base of a pile to "shore" up the wall and add interest.
If you can obtain a copy of the book The Verlinden Way Vol. 2 it details how to make sandbags from putty and how to paint and detail them as well.
I hope this helps. I look forward to seeing your completed project here on the forum soon.
Cheers;
Gregory